Completed

Problem_1.py

@@ -0,0 +1,44 @@
+class Solution:
+    def solveNQueens(self, n: int) -> List[List[str]]:
+        sol = []
+        board = [[False for i in range(n)] for i in range(n)]
+        cols = set()
+        diags = set()       # row - col
+        anti_diags = set()  # row + col
+
+        def recur(row):
+            if row == n:
+                temp = []
+                for i in range(n):
+                    s = ''
+                    for j in range(n):
+                        if board[i][j]:
+                            s = s + 'Q'
+                        else:
+                            s = s + '.'
+                    temp.append(s)
+                sol.append(temp)
+                return
+
+            for i in range(n):
+                # O(1) conflict check instead of O(n) loops!
+                if i in cols or (row - i) in diags or (row + i) in anti_diags:
+                    continue
+
+                # Place queen
+                board[row][i] = True
+                cols.add(i)
+                diags.add(row - i)
+                anti_diags.add(row + i)
+
+                recur(row + 1)
+
+                # Backtrack - remove queen
+                board[row][i] = False
+                cols.remove(i)
+                diags.remove(row - i)
+                anti_diags.remove(row + i)
+
+        recur(0)
+        return sol
+

Problem_2.py

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+# // Time Complexity : O(2^n)
+# // Space Complexity : O(n)
+class Solution:
+    def exist(self, board: List[List[str]], word: str) -> bool:
+        n = len(board)
+        m = len(board[0])
+
+        def recur(i, j, wrd_idx):            
+            if wrd_idx == len(word):
+                return True
+
+            if i < 0 or i >= n or j < 0 or j >= m or board[i][j] != word[wrd_idx]:
+                return False
+
+            temp = board[i][j]
+            board[i][j] = '#'  
+
+            found = (recur(i-1, j, wrd_idx+1) or 
+                    recur(i+1, j, wrd_idx+1) or 
+                    recur(i, j-1, wrd_idx+1) or 
+                    recur(i, j+1, wrd_idx+1))
+
+            board[i][j] = temp
+            return found
+
+        for i in range(n):
+            for j in range(m):
+                if recur(i, j, 0):
+                    return True
+
+        return False